Could someone help me how to connect/build the bridge: preamp unballanced output -&gt;transformer-&gt;ballanced long connection-&gt;transformer-&gt;unbalanced input of power amp using those 4 trannies ?

S&B offer next to no useful information about either of those transformers so there's not much I can offer regarding those specific devices.

However I like to keep things simple so here's what I do.

I don't bother converting unbalanced outputs to balanced outputs. Instead, I just use a Jensen JT-11P-1 input transformer at my amplifier's input.

The reason for this is that the JT-11P-1 will give you 100dB of common-mode noise rejection even from an unbalanced source. In fact, it only gives about 7dB more if it's fed from a balanced source. And I haven't found that 7dB to make any difference at all in the end.

So if the TX-101 has similar performance to the JT-11P-1, I'd recommend returning the output transformers and just install the 101s as close as possible to the inputs of your amps. Ideally build them into the chassis if you can.

Originally posted by mrfeedback Hi Vladimir,
You also need to ensure that the secondaries are loaded correctly, or else you will get a non-flat frequency response.

Care to elaborate Steve ?.

Well there's not much to elaborate on given that S&B don't offer much useful information about their transformers.

But yes, to avoid high frequency resonant peaks, you need to make sure that the transformer is properly loaded. For example, the JT-11P-1 is ideally loaded by 10k ohms.

If you're using output transformers, you also have to concern yourself with source capacitance.

Output transformers typically have significantly lower primary inductance than input transformers. So if the output stage feeding it is capacitively coupled, you need to make sure you have enough capacitance to avoid low frequency resonances (caused by the output capacitor and the primary inductance of the transformer).

For most output transformers, the few microfarads of output coupling capacitance that is typical isn't high enough. You usually need to have at least several hundred microfarads of coupling capacitance. This pretty much forces you to use electrolytics and is another reason to not bother with output transformers and stick to input transformers only.

By the way what do you recommend for an output transformer, and have you tried Balsa wood to control vibrations?

The JT-11-DMCF is a good output transformer as far as output transformers go. But still I prefer to use just an input transformer on its own. Also see my comments above regarding output transformers and low frequency resonances.

As for balsa wood, I played with it a bit quite a few years ago but it didn't seem to work well and the stuff just falls apart too easily.

For metal chassis, the best stuff I've used are the various products from E*A*R. For wood, I prefer natural cork (as opposed to the cork/rubber blends).